An organic EL (Electroluminescence) display device is represented as an example of a display device. An organic EL display device has a plurality of organic light-emitting elements (hereinafter, referred to as a light-emitting element) formed over a substrate, and each light-emitting element possesses an electroluminescence layer (hereinafter, referred to as an EL layer) including an organic compound between a pair of electrodes (cathode and anode) as a fundamental structure. Holes and electrons are respectively supplied to the EL layer from the anode and the cathode when a potential difference is provided between the pair of electrodes. The holes and electrons are recombined, resulting in an excited state of the organic compound. Light emission in radiative deactivation of the excited state to a ground state is utilized to realize a function as a light-emitting element.
Efficiency and emission color of a light-emitting element are controlled by the structure of the EL layer and an emission material included in the EL layer. For example, it is possible to obtain a variety of emission colors by appropriately selection of an emission material. In addition, the use of light-interference effect in or outside a light-emitting element increases emission intensity in a front direction and narrows an emission spectrum. For example, it is disclosed in Japanese Patent Application Publication No. 2000-323277 that a thickness of a hole-transporting layer included in an EL layer is adjusted in every light-emitting element, which controls a resonance structure formed by the EL layer and the pair of electrodes. By this method, emission intensity and emission color can be optimized in every light-emitting element.